(a) Field of the Invention
The present invention relates to an electro-acoustic transducer of the so-called electro-dynamic type for use in, for example, loudspeakers, earphones and microphones, and more particularly it pertains to a planar type electro-acoustic transducer designed to impart a substantially uniform vibration to the entire areas of a vibratable diaphragm in case the present invention is applied to a loudspeaker and an earphone.
(b) Description of the Prior Art
As is known well, a basic structure of the so-called planar type electro-acoustic transducer, in general, is comprised of a vibratable diaphragm having a vibratable portion and a fixed stationary portion, a voice coil formed in the vibratable portion of the diaphragm, and a magnet plate having the formations of opposite magnetic poles on a surface thereof for applying to said voice coil such magnetic field as will ensure that the directions of the vector products thereof with the electric current flowing through the voice coil are rendered to be uniform in all parts of the voice coil. The magnet plate assembly has two types, one of which is comprised of two magnet plate members sandwiching the diaphragm therebetween while keeping appropriate air gaps between the surfaces of the respective magnet plates and the diaphragm carrying a coil on each side thereof. The other type comprises a magnet plate member disposed at a coil-carrying side of the diaphragm with an air gap therebetween. The former is called the push-pull type, and the latter is called the single type magnet plate assembly.
An example of a conventional planar type electro-acoustic transducer is shown in Japanese Utility Model Preliminary Publication No. Sho 51-100536, as shown in FIG. 1, which is comprised of a disk-shaped diaphragm 3 having thereon a voice coil formed with a helical winding of a bundle 2 of electric conductor 1 of a given length having at least two circular turns and a disk-shaped magnet plate 4. There is an advantage in this example that a powerful diaphragm-driving force can be obtained even when the interval between the opposite magnetic poles produced on the free surface of the magnet plate 4 is set relatively large as shown in FIG. 2, since a number of individual conductor strips through which electric current flows in a same direction are provided in the regions located between the respective opposite magnetic poles. Also, there is a further advantage that, owing to the fact that these strips of electric conductor are connected in series to each other, the impedance of this voice coil can be set at such a large value as is required for microphones and like devices.
In said conventional electro-acoustic transducer, however, there is a problem that the magnetic fluxes of the magnet no longer cross at right angle the signal-carrying electric current which flows through the voice coil even if the centers of the voice coil and the magnetic poles of the magnet plate have been registered precisely relative to each other, if there is present any slight positional misalignment in the circumferential direction of the helical pattern of the voice coil relative to the helical pattern of the magnetic poles when the disk-shaped diaphragm 3 and the disk-shaped magnet plate 4 are assembled together while paying attention to have these two members positioned so as to face each other properly. As a result, the efficiency of this electro-acoustic transducer drops markedly. That is, in case such electro-acoustic transducer is applied to a loudspeaker or an earphone, the transducer not only will exhibit a degradation of its output quality, but also would cause the generation of partial vibration of the planar type diaphragm. Also, in case such transducer is applied to a microphone, the induced electromotive force is partly cancelled out, so that the output of this transducer drops substantially. For these reasons, there has been required for an extremely precise positioning of the diaphragm and the magnet when they are assembled together into a complete transducer, and this has been a quite troublesome work and has led to an increase in the cost of manufacture.